1. Field of the Invention
This invention relates to a positive resist composition which lends itself to a lift-off technique.
2. Prior Art
In the prior art, dry and wet etching processes are commonly used in forming conductor patterns such as aluminum electrodes on semiconductor substrates. The process is illustrated in FIG. 3 as comprising metal layer sputtering and resist patterning to form a resist pattern 3 on a metallization layer 2 on a substrate 1. The exposed area of the metallization layer 2, that is, those metallization portions which are not covered with the resist pattern 3 are etched away. By treating the structure with a stripping solution, the resist pattern 3 is stripped off to leave a conductor pattern 2 on the substrate 1.
The above-mentioned process, however, suffers from low processing precision and a problem associated with etching procedure where difficult-to-etch metals such as gold and tantalum are used as the metallization. To avoid such inconvenience, a lift-off technique is often used in these years. The lift-off technique is illustrated in FIG. 4 as comprising resist patterning to form a resist pattern 3 on a substrate 1, metal sputtering to form a metal layer 2 on the resist pattern 3 and substrate 1, and processing the structure with a stripping solution to strip or lift off the resist. A conductor pattern can be formed without an etching procedure. This lift-off technique ensures precision processing since a metal pattern is formed using a precisely configured resist pattern as a template. Even a difficult-to-etch metal can be readily processed due to the eliminated need for etching as long as it can be deposited on the substrate and resist layer. The resist stripping step plays the most important role in the lift-off technique. If the resist can be effectively stripped in this step without leaving residues, the lift-off technique is expected to accomplish improvements in processing precision and reliability.
However, where patterns are formed by the lift-off technique using conventional positive resist compositions, the resist stripping step does not effectively take place and can leave resist residues, arising a problem of reliability.
Therefore, the lift-off technique often employs a step requiring complex, cumbersome operations, for example, a dual layer resist process and an image reversal resist process.
JP-A 69111/1996 (U.S. Ser. No. 573,578) discloses a resist material from which a pattern configuration suitable for the lift-off technique is obtained by a simple single-layer positive resist process. This resist material is epoch-making in that a pattern configuration suitable for the lift-off technique is obtained by an ordinary single-layer positive type one-step resist process while the resist material is prepared from three components, a novolak resin, a photosensitive agent, and a dissolution promoter. Then, in order to accomplish high resolution, dimensional control, heat resistance and film retention which are believed to be increasingly required in the future lift-off involving process, various contrivances concerning both the novolak resin and the photosensitive agent are necessary like conventional novolak base positive resist materials.
One exemplary measure associated with the novolak resin side is described. If a novolak resin is synthesized to a lower average molecular weight, the resulting resist layer is improved in resolution, but becomes poor in heat resistance and film retention. It then becomes necessary to take an appropriate measure of removing a low molecular weight novolak fraction as by subjecting the synthesized novolak resin to re-precipitation. The following measure is associated with the photosensitive agent side. As the light source of an aligner shifts from g-line to i-line, light is not easily transmitted by a resist layer containing a conventionally used benzophenone photosensitive agent. It then becomes necessary to take an appropriate measure of using a non-benzophenone photosensitive agent causing less absorption of i-line.
However, in order that a resist material for the lift-off technique having higher resolution as well as satisfactory dimensional control, heat resistance, and film retention be accomplished by combining such measures, many additional steps are necessary in the resist preparation process and undesirably cause a cost increase.
Therefore, an object of the present invention is to provide a positive resist composition for use in the lift-off technique which has solved the above-mentioned problems of the lift-off technique, which can form a reliable conductor pattern at high processing precision, which is sufficiently heat resistant to withstand the elevated temperature encountered upon metal sputtering, and which can be prepared by a simple process. Another object of the present invention is to provide a method for forming a wiring or conductor pattern using the resist composition according to the lift-off technique.
Although the lift-off technique generally needs complex steps using a dual-layer resist or image reversal resist, the invention intends to provide a positive resist composition from which a resist pattern having high working precision and reliability and amenable to the lift-off technique can be formed through a single-layer positive type one-step resist process. The resultant resist pattern has sufficient heat resistance to withstand the heat encountered upon metal sputtering, can be formed by simple steps, and can be advantageously used in forming a wiring pattern such as an electrode of a difficult-to-etch metal.
In search of a resist composition for use in the lift-off technique which has solved the above-mentioned problems of the prior art and has the advantages of high resolution, dimensional control, heat resistance and film retention, we have found that by blending (A) a 1,2-naphthoquinonediazidosulfonyl-introduced novolak resin containing at least one recurring unit of the following general formula (1) and having a weight average molecular weight calculated as polystyrene of 2,000 to 20,000, 2.5 to 27 mol % of the hydrogen atom of a hydroxyl group in the novolak resin being replaced by a 1,2-naphthoquinonediazidosulfonyl group, with (B) a low molecular aromatic compound having phenolic hydroxyl groups and 2 to 20 benzene rings, the ratio of the number of phenolic hydroxyl groups to the number of benzene rings being from 0.5 to 2.5, the low molecular aromatic compound being represented by the following general formula (2) or (3), to thereby form a positive resist composition, applying the resist composition onto a substrate to thereby form a resist layer, preferably baking the resist layer at a temperature of 90 to 130xc2x0 C. before exposure or development, and effecting exposure and development, there can be formed a resist pattern which has an undercut of desired configuration and is sufficiently improved in resolution, dimensional control, heat resistance and film retention to lend itself to the lift-off technique. 
In formula (1), m is an integer of 0 to 3. 
In formula (2), m is an integer of 0 to 2, n is an integer of 0 to 2, with the proviso that m is equal to 1 or 2 when n is equal to 0. R1 to R6 are independently a hydrogen atom, a methyl group, a group of the following formula (4) or a group of the following formula (5). A is hydrogen, methyl or a group of the following formula (4) when n is 0 and m is 1; one A is methylene or a group of the following formula (6) and the other A is hydrogen, methyl or a group of the following formula (4) when n is 0 and m is 2; A is methylene or a group of the following formula (6) when n is 1; A is methine or a group of the following formula (7) when n is 2 and m is 1; and one A is methylene or a group of the following formula (6) and the other A is methine or a group of the following formula (7) when n is 2 and m is 2.
In formula (3), h is a sufficient number to give a weight average molecular weight of 200 to 2,500. 
In formulae (4) to (7), p, q, r, s, and t each are an integer of 0 to 3.
More particularly, in the resist composition amenable to the lift-off technique according to the invention, the novolak resin having a photosensitive agent integrated therewith maintains a low average molecular weight and high resolution until exposure, but converts into a higher molecular weight one after development because the novolak resin undergoes crosslinking reaction through azo-coupling reaction during development. As a result, the pattern formed from the resist composition has very high heat resistance. Therefore, the invention ensures high resolution and heat resistance at the same time.
Since the matrix of the photosensitive agent is a novolak resin itself according to the invention, it is unnecessary to change the photosensitive agent even when the wavelength changes with different light sources. Then not only a resist composition featuring high resolution and heat resistance can be produced through simple steps at low cost, but also a universal resist composition is available independent of the wavelength of the light source of various aligners.
Additionally, since the composition of the invention consists essentially of two components, (A) an integrated resin serving as an alkali-soluble resin and a photo-sensitive agent and (B) a dissolution promoter, baking before or after exposure is likely to induce segregation of the dissolution promoter in proximity to the substrate. Then an undercut of desired configuration and dimensions suitable for use in the lift-off technique can be developed in the resist pattern as desired.
Therefore, according to the invention, a resist pattern having an undercut of desired configuration suitable for use in the lift-off technique and featuring high resolution, heat resistance and dimensional stability can be formed, whereby a wiring pattern featuring high processing precision and reliability can be formed by the lift-off technique.
Accordingly, in a first aspect, the present invention provides a positive resist composition for use in the lift-off technique comprising in admixture, (A) a 1,2-naphthoquinonediazidosulfonyl-introduced novolak resin containing at least one recurring unit of the general formula (1) and having a weight average molecular weight calculated as polystyrene of 2,000 to 20,000, 2.5 to 27 mol % of the hydrogen atom of a hydroxyl group in the novolak resin being replaced by a 1,2-naphthoquinonediazidosulfonyl group, and (B) a low molecular aromatic compound of the general formula (2) or (3) having phenolic hydroxyl groups and 2 to 20 benzene rings, the ratio of the number of phenolic hydroxyl groups to the number of benzene rings being from 0.5 to 2.5.
In a second aspect, the present invention provides a method for forming a pattern comprising the steps of:
forming a resist layer on one surface of a substrate,
processing the resist layer to form a resist pattern,
metallizing the substrate surface including the resist pattern, and
stripping off the resist pattern to leave a pattern of metallization on the substrate,
wherein the resist layer constituting said resist pattern is formed with an undercut.
Preferably, in a profile of said resist layer constituting said resist pattern as shown in FIGS. 1 and 2 wherein the resist layer has a line width of L xcexcm and a thickness of T xcexcm, and the undercut has a cut height of A xcexcm and a cut depth of B xcexcm, the resist layer has a thickness T of up to 20 xcexcm and the undercut has an extent of cut defined by the equations (I) and (II):
1/20xe2x89xa6A/Txe2x89xa62/5 xe2x80x83xe2x80x83(I)
1/20xe2x89xa6A/Bxe2x89xa64xe2x80x83xe2x80x83(II).
Preferably, the resist layer is constructed of the positive resist composition of the first aspect.
More preferably, the steps of forming a resist layer on one surface of a substrate and processing the resist layer to form a resist pattern include
forming a resist layer on one surface of a substrate from the positive resist composition of the first aspect,
baking the resist layer at a temperature of 90 to 130xc2x0 C. before exposure or before development,
exposing the resist layer to light,
developing the resist layer, thereby forming an undercut in the resist layer.